Precisely vertical and horizontal rifle scope crosshairs, or reticles, become increasingly important to shot accuracy as the downrange distance increases. A slight cant, or angle error, in the crosshairs can cause noticeable error or missed shots at long distances. A cant of five degrees may cause an error of 3.7 inches at 300 yards.
Stated another way, the firearm barrel axis, along which the projectile travels, must lie in a common vertical plane with the scope axis, along which the shooter sights the target. However, when the firearm is properly sighted in, the barrel axis is not parallel to the scope axis. The barrel axis must be tilted with the muzzle up (elevation) in order to counteract gravitational drop of the projectile during its flight time to the target. However, the scope axis is usually truly horizontal. When the barrel and scope axes lie in a non-vertical plane, the barrel axis elevation angle causes the projectile to miss low to the right or left of the target.
Numerous devices have been developed with the goal of aligning a firearm barrel axis and a scope axis precisely in a common vertical plane and aligning the vertical and horizontal scope reticles to be truly vertical and horizontal. Many of these devices incorporate spirit levels. Some examples include the Weaver Crosshair Leveling Kit, the Straight Shot Segway Reticle Kit and the Wheeler Pro Reticle Leveling Kit. There are at least two shortcomings with devices such as these: 1) inadequate means for mounting the devices to the firearm or scope and 2) low precision fabrication of the devices, in particular low sensitivity spirit levels.
The means for mounting the devices to the firearm or scope may be inadequate for several reasons. Devices may make contact with firearm or scope features or surfaces which are not reliably and precisely oriented with respect to the barrel axis, the scope axis, and/or the vertical plane for alignment. Devices may be fabricated with compliant parts, such as a magnetic pad, for primary contact with the firearm or scope. Devices may have relatively small contact areas or contact lengths with the firearm or scope features.
Devices may be fabricated with conventional industry tolerances for production parts, or in some instances tolerances that may be wider than industry standard. In an assembly, the tolerances may stack up unfavorably, resulting in low precision.
There is a need for apparatus and methods which overcome these drawbacks by incorporating means for mounting the apparatus to reliable precision machined surfaces of the firearm or scope, which surfaces have known, consistent orientations relative to the barrel axis, the scope axis, and/or the vertical plane for alignment. There is also a need for apparatus and methods with high precision, for example components fabricated and assembled to gage makers tolerances, and high sensitivity spirit levels.